Why it’s still around: Throughout the 1990s, the U.S. switched from using a vaccine that contained the entire bacterium (which posed a higher risk of side effects) to a less potent "acellular" version. Experts now suspect the newer vaccine is wearing off faster.

What you can do: Talk to your doctor about a booster shot of Tdap (for tetanus, diphtheria and pertussis), especially if you haven't had one in 10 years, you're in contact with infants or you're pregnant. The "whoop" occurs when the infected person tries to suck in air to fill their lungs, but you can be sick without the sound, says Alison Patti, a spokeswoman for the CDC's bacterial diseases division. Watch for a cough that gets worse at night and won't go away (in China, pertussis is known as "the 100 days' cough"). If you think you have it, make an appointment to see your doctor, because antibiotics are most effective when taken early.

Why it’s still around: Thanks to MMR vaccinations (protecting against measles, mumps and rubella), the United States had all but eradicated measles. Unfortunately, the virus still kills nearly 200,000 people around the world, and it's managed to hitch a ride with travelers and infect vulnerable and unvaccinated people here in the U.S.